In some cases, timezone_name_from_abbr() may return a "historical" (i.e. deprecated) timezone identifier rather than the current standard one for a given location. For example:<?phpecho timezone_name_from_abbr('EASST'); // prints "Chile/EasterIsland" instead of "Pacific/Easter"?>

This means that the returned timezone identifier is not guaranteed to be in the results of timezone_identifiers_list() unless you include the "backwards compatible" timezones.<?phpvar_dump(in_array(timezone_name_from_abbr('EASST'), timezone_identifiers_list())); // (bool)falsevar_dump(in_array(timezone_name_from_abbr('EASST'), timezone_identifiers_list(DateTimeZone::ALL_WITH_BC))); // (bool)true?>